JP-2005-120874A (U.S. Pat. No. 7,146,944B2) shows an electric valve timing controller equipped with a drive circuit which controls the energization to the electric motor based on a target rotation speed and an actual rotation speed of the electric motor so as to rotates the electric motor to a target rotation direction.
JP-5-22979A indicates a well known technology in which the electric motor rotates in one way direction. However, it is difficult to apply this technology to the electric valve timing controller which rotates the electric motor to a normal rotation direction and a reverse rotation direction.
A research has been was conducted under the above background about the electric valve timing controller which rotates the electric motor in both directions, and the technology which performs feedback control of the energization to the electric motor using the map shown in FIG. 13A is developed.
Specifically in this technology, a difference is computed by converting the target rotation speed and the actual rotation speed on individual map. The energization instruction value of the electric motor is determined based on this difference value. As shown in FIG. 13A, a map used for conversion of the target rotation speed and the actual rotation speed defines the conversion relationship which offsets the output y representing the normal direction and the reverse direction with respect to an input rotation speed x of the zero value. Here, the offset in the map is established based on the following reason. In a valve timing controller provided with an electric motor, the electric motor is driven in the same direction or the reverse direction as the rotation direction of the engine in order to adjust the valve timing. While the valve timing is maintained, the electric motor is driven in the normal rotation direction as the same speed as the internal combustion engine. In order to prevent the situation where the actual rotation speed of the electric motor shifts from the target rotation speed by friction loss during this period, it is necessary to always perform a certain amount of energization to the electric motor.
According to the further study about the technology which uses the map, the following problem is found. When changing the target rotation direction and making it differ from the actual rotation direction, the amount of energization corresponding to the sum of offset amount Δ of the output y is superfluously needed, so that energy consumption increases. Then, it is considered that a map with the offset and a map without offset as shown in FIG. 13B are respectively used between a case where the target rotation direction is unchanged and a case where the target rotation direction is changed. However, it is apparent that the drive circuit is complicated.
The present invention is made in view of such a problem, and an object of the present invention is to provide an electric valve timing controller which simplifies processing required for rotation of the electric motor.